A diminished visual acuity or total loss of vision may result from a number of eye diseases or disorders caused by dysfunction of tissues or structures in the anterior segment of the eye and/or posterior segment of the eye. Of those that occur as a consequence of a dysfunction in the anterior segment, aberrations in the visual cycle are often involved. The visual cycle (also frequently referred to as the retinoid cycle) comprises a series of light-driven and/or enzyme catalyzed reactions whereby a light-sensitive chromophore (called rhodopsin) is formed by covalent bonding between the protein opsin and the retinoid agent 11-cis-retinal and subsequently, upon exposure to light, the 11-cis-retinal is converted to all-trans-retinal, which can then be regenerated into 11-cis-retinal to again interact with opsin. A number of visual, ophthalmic, problems can arise due to interference with this cycle. It is now understood that at least some of these problems are due to improper protein folding, such as that of the protein opsin.
The main light and dark photoreceptor in the mammalian eye is the rod cell, which contains a folded membrane containing protein molecules that can be sensitive to light, the main one being opsin. Like other proteins present in mammalian cells, opsin is synthesized in the endoplasmic reticulum (i.e., on ribosomes) of the cytoplasm and then conducted to the cell membrane of rod cells. In some cases, such as due to genetic defects and mutation of the opsin protein, opsin can exhibit improper folding to form a conformation that either fails to properly insert into the membrane of the rod cell or else inserts but then fails to properly react with 11-cis-retinal to form native rhodopsin. In either case, the result is moderate to severe interference with visual perception in the animal so afflicted.
Among the diseases and conditions linked to improper opsin folding is retinitis pigmentosa (RP), a progressive ocular-neurodegenerative disease (or group of diseases) that affects an estimated 1 to 2 million people worldwide. In RP, photoreceptor cells in the retina are damaged or destroyed, leading to loss of peripheral vision (i.e., tunnel vision) and subsequent partial or near-total blindness.
In the American population the most common defect occurs as a result of replacement of a proline residue by a histidine residue at amino acid number 23 in the opsin polypeptide chain (dubbed “P23H”), caused by a mutation in the gene for opsin. The result is production of a destabilized form of the protein, which is misfolded and aggregates in the cytoplasm rather than being transported to the cell surface. Like many other protein conformational diseases (PCDs), the clinically common P23H opsin mutant associated with autosomal dominant RP is misfolded and retained intracellularly. The aggregation of the misfolded protein is believed to result in photoreceptor damage and cell death.
Recent studies have identified small molecules that stabilize misfolded mutant proteins associated with disease. Some of these, dubbed “chemical chaperones,” stabilize proteins non-specifically. Examples of these include glycerol and trimethylamine oxide. These are not very desirable for treating ophthalmic disease because such treatment usually requires high dosages that may cause toxic side effects. Other agents, dubbed “pharmacological chaperones,” (which include native ligands and substrate analogs) act to stabilize the protein by binding to specific sites and have been identified for many misfolded proteins, e.g., G-protein coupled receptors. Opsin is an example of a G-protein coupled receptor and its canonical pharmacological chaperones include the class of compounds referred to as retinoids. Thus, certain retinoid compounds have been shown to stabilize mutant opsin proteins (see, for example, U.S. Patent Pub. 2004-0242704, as well as Noorwez et al., J. Biol. Chem., 279(16): 16278-16284 (2004)).
The visual cycle comprises a series of enzyme catalyzed reactions, usually initiated by a light impulse, whereby the visual chromophore of rhodopsin, consisting of opsin protein bound covalently to 11-cis-retinal, is converted to an all-trans-isomer that is subsequently released from the activated rhodopsin to form opsin and the all-trans-retinal product. This part of the visual cycle occurs in the outer portion of the rod cells of the retina of the eye. Subsequent parts of the cycle occur in the retinal pigmented epithelium (RPE). Components of this cycle include various enzymes, such as dehydrogenases and isomerases, as well as transport proteins for conveying materials between the RPE and the rod cells.
As a result of the visual cycle, various products are produced, called visual cycle products. One of these is all-trans-retinal produced in the rod cells as a direct result of light impulses contacting the 11-cis-retinal moiety of rhodopsin. All-trans-retinal, after release from the activated rhodopsin, can be regenerated back into 11-cis-retinal or can react with an additional molecule of all-trans-retinal and a molecule of phosphatidylethanolamine to produce N-retinylidene-N-retinylethanolamine (dubbed “A2 E”), an orange-emitting fluorophore that can subsequently collect in the rod cells and in the retina pigmented epithelium (RPE). As A2E builds up (as a normal consequence of the visual cycle) it can also be converted into lipofuscin, a toxic substance that has been implicated in several abnormalities, including ophthalmic conditions such as wet and dry age related macular degeneration (ARMD). A2E can also prove toxic to the RPE and has been associated with dry ARMD.
Because the build-up of toxic visual cycle products is a normal part of the physiological process, it is likely that all mammals, especially all humans, possess such an accumulation to some extent throughout life. However, during surgical procedures on the eye, especially on the retina, where strong light is required over an extended period, for example, near the end of cataract surgery and while implanting the new lens, these otherwise natural processes can cause toxicity because of the build-up of natural products of the visual cycle. Additionally, excessive rhodopsin activation as a result of bright light stimulation can cause photoreceptor cell apoptosis via an AP-1 transcription factor dependent mechanism. Because of this, there is a need for agents that can be administered prior to, during or after (or any combination of these) the surgical process and that has the effect of inhibiting rhodopsin activation as well as reducing the production of visual cycle products that would otherwise accumulate and result in toxicity to the eye, especially to the retina.
The present invention answers this need by providing small molecules which noncovalently bind to opsin or mutated forms of opsin for treating and/or amelioration such conditions, if not preventing them completely. Importantly, such agents are not natural retinoids and thus are not tightly controlled for entrance into the rod cells, where mutated forms of opsin are synthesized and/or visual cycle products otherwise accumulate. Therefore, such agents can essentially be titrated in as needed for facilitating the proper folding trafficking of mutated opsins to the cell membrane or prevention of rhodopsin activation that can lead to the excessive build-up of visual cycle products like all-trans-retinal that in turn can lead to toxic metabolic products. Such compounds may compete with 11-cis-retinal to reduce all-trans-retinal by tying up the retinal binding pocket of opsin to prevent excessive all-trans-retinal build up. Thus, the compounds provided by the present invention have the advantage that they do not directly inhibit the enzymatic processes by which 11-cis-retinal is produced in the eye (thus not contributing to retinal degeneration). Instead, the formation of all-trans-retinal is limited and thereby the formation of A2E is reduced. Finally, by limiting the ability of 11-cis-retinal to combine with opsin to form rhodopsin, rhodopsin activation caused by bright light stimulation especially during ophthalmic surgery is also diminished thus preventing the photocell death that results.
Mislocalization of photoreceptor cell visual pigment proteins (opsins) can occur in various ocular diseases, and also with normal aging. In both cases the accumulation of mislocalized opsin leads to the decline in viability of photoreceptor cells. With time this mislocalized opsin accumulation leads to rod and cone cell death, retinal degeneration, and loss of vision. The present invention solves this problem by providing a method of correcting mislocalized opsin within a photoreceptor cell by contacting a mislocalized opsin protein with an opsin-binding agent that binds reversibly and/or non-covalently to said mislocalized opsin protein, and promotes the appropriate intracellular processing and transport of said opsin protein. This correction of mislocalization relieves photoreceptor cell stress, preventing decline in viability and death of photoreceptor cells in various diseases of vision loss, and in normal age-related decline in dim-light and peripheral rod-mediated vision, central cone-mediated vision, and loss of night vision.